Incandescent lamp with a glower made of an alloyed semiconductor material



'March 24, 1970 M. v. FOK 3,502,930

INCANDESCENT LAMP WITH A GLOWER MADE OF AN ALLOYED SEMICONDUCTOR MATERIAL Filed Feb. 17. 1967 2 Sheets-Sheet 1 I 0 1 2 3hv(ev) kg f H52 0 7 2 3hv(ev) E 1 0 1 '2 3 hv(ev) INCANDESGENT LAMP WITI-i A GLOWER MAD-E OF AN ALLOYED SEMICONDUCTOR MATERIAL March 2 4, 1970 M v FOK- 3,502,930

Filed Fgb. 17, 1967 2 Sheets-Sheet 2 United States Patent 3,502,930 INCANDESCENT LAMP WITH A GLOWER MADE OF AN ALLOYED SEMICONDUCTOR MATERIAL Mikhail Vladimirovich Fok, Ulitsa Gorkogo 8, kv. 103, Moscow, U.S.S.R. Filed Feb. 17, 1967, Ser. No. 616,887 Int. Cl. H01j 17/04 US. Cl. 313218 9 Claims ABSTRACT OF THE DISCLOSURE electron volt.

The present invention relates to electric incandescent lamps with semiconductor glowers.

Known in the prior art are incandescent lamps with tungsten filaments. They have a low luminous efiiciency as the main power of emission of the tungsten filament falls Within the infrared region rather than within the visible one. This takes place because the absorption capacity of tungsten is very small and depends upon the wavelength and because, according to Kirchhoffs law, its luminosity spectrum, when heated, is very close to that of the black body.

There are two ways to increase the luminous efiiciency of incandescent lamps: to raise the filament temperature and to use selective emitters.

The first way is characterized in that the maximum in the black body emission spectrum, according to Wiens law, is displaced into the shortwave region as the temperature rises. That is why at relatively low temperatures the portion of visible radiation in the black body will rise. The largest portion of the visible radiation in the black black body spectrum will be when the temperature thereof reaches 7000 K. A further temperature rise will only reduce the portion of the visible radiation due to the speedy rise in the portion of ultraviolet radiation. At 7000 K. no substance can remain solid. Moreover, if it were possible to create a black emitter with such a temperature the portion of visible radiation in the most favorable case would amount to 37% of all emitted power. However, this meets large difficulties connected with the unstability of the glower at high temperatures. At present in lamps with iodine cycle the filament temperature hardly reaches 3200 K.

The second way is characterized in that the glower is r made of a substance which is transparent in the infrared spectrum region. According to Kirchhoifs law such a glower emits nothing in this region, i.e. all the radiation lies within the visible (and partially in the ultraviolet) spectrum portion. The luminous efliciency of the lamp with such a glower will be very high. There remains only to develop a substance which would be suitable for the manufacture of such glowers.

It is known that such a substance may be a sufficiently heat-resistant, chemically pure semiconductor with a wide forbidden gap (J. Optics and Spectroscopy, vol. 13, p. 612, 1962, Moscow).

However, a lamp with a glower made of such a semiconductor will have a series of disadvantages.

In such a lamp it will be extremely diflicult to obtain stable heating conditions for the glower. To this effect it 3,502,930 Patented Mar. 24, 1970 is necessary to make the glower either very thin in order to reduce the infrared radiation by free electrons or to considerably increase the working temperature in order to make the temperature dependence of the glower conductivity sufiiciently low.

As the pure semiconductor with a wide forbidden gap has a very high resistance at room temperature, it may be heated with a current passing therethrough only in such a case if it has been heated to a sufficiently high temperature. That is why such a lamp requires a powerful starting device which will heat the glower to a temperature of about 1000 K. Moreover, the manufacture of reasonably pure, thermally stable conductors with sufliciently large dimensions presents problems.

An object of the present invention is to eliminate the above-said disadvantages and to provide an electric incandescent lamp with a glower made of a semiconducting material ensuring a sufiiciently high luminous efiiciency, stability of heating conditions and the independent ignition of the lamp beginning from room temperature or one lower one.

These and other objects of the invention are achieved due to the fact that the material of the glower comprises a semiconductor so alloyed with admixtures that even at such a temperature at which the glower operates, the conductivity of the semiconductor remains admixing. For such a purpose either donor or acceptor admixtures with concentrations about 10 to 10 cmr may be used. However, to introduce both donor and acceptor admixtures simultaneously is not recommended.

If it is desirable for the lamp to be ignited independently at room temperature or lower temperature without additional heating, such admixtures are employed which will contribute to levels in the semiconductor not deeper than 0.5 ev. from the conduction band for donor admixtures or from the valence band for the acceptor ones. If independent ignition is required the admixtures are to be taken which can provide deeper levels, say, to 0.3 ev.

For the material of a glower use may be made of alloyed Carborundum. Alloyed with nitrogen or phosphorus it will ensure the independent ignition of the lamp. As alloying admixtures, aluminium and boron may be also used.

In order to reduce thermal losses due to thermal conduction, the lamp bulb may be made with double walls on the principle of the Dewar flask. If such a flask is made of glass it may increase the luminous efiiciency to approximately 10%. If it is made of quartz, the luminous efficiency of the lamp may be increased to 20% as against the semiconductor lamp with the usual bulb.

The invention will be illustrated hereinbelow by way of example, with reference to the accompanying drawing, in which:

FIGS. 1, 2, 3 show glower luminosity spectra of the lamp with a semiconductor glower at different temperatures; and

FIG. 4 is a diagrammatic section taken through an incandescent lamp according to the invention.

Let us consider the principle of operation of semiconductor incandescent lamps, which does not depend upon whether the glower is made of pure or alloyed semiconductor.

In FIGS. 1, 2, 3 the abscissae represent the quantum energy h of the radiation and the ordinates-the power '15 of the radiation in arbitrary units. The solid line shows the luminosity spectrum of the semiconductor glower and the dotted line shows the black body spectrum. The luminosity spectrum of the glower made of semiconductor in the infrared region A is marked with inclined shading and the same in the visible region B-with vertical shadmg.

As can be seen from the diagrams, the emission of the semiconductor glower in the infrared region is far less than the emission of the black body because in this region the semiconductor is transparent. FIGS. 1, 2, 3 show successively rising glower temperatures. Comparing these curves one can see that on raising the temperature, the portion of emission falling within the visible region constantly changes and the region of the semiconductor transparency reduces due to reduction of the forbidden gap width as the heating progresses. The short wave boundary C (FIG. 1) being in the visible spectrum portion, the rise in the temperature leads to the more rapid increase in the power of radiation in the visible region as compared with the whole radiated power. The power radiated in the ultraviolet spectrum portion increases far more rapidly, but at temperatures of 3,000 K. it is rather small in absolute value and unessential. When the boundary of the transparency region spreads into the infrared spectrum portion (C and C FIGS. 2 and 3), the infrared radiation drastically increases. It will soon result in a drop in the luminous efliciency of the glower. Thus, the optimal temperature for a glower made of semiconductor material is less than that for the black body. The maximum efficiency of the glower thereby may be higher than that of the black body.

The radiation in the semiconductor transparency region, according to Kirchhoffs law, is defined by the absorption capacity of the glower in this region. The increase in the absorption leads to the decrease in the efficiency. The maximum efliciency is shifted thereby to more higher temperatures. Thus, it is desirable that the absorption capacity in the transparency region be as small as possible. This means that the glower should be as thin as possible. Calulations show that the role of the glower thickness increases as the semiconductor transparency region expands towards the side of long waves. On the other hand, the absorption capacity is also defined by the nature and concentration of admixtures in the semiconductor. Calculations show that the depth of levels formed by the donor or acceptor admixture should not surpass 0.5 ev. from the appropirate band otherwise the glower emission in the infrared region will prohibitively increase.

Calculations also show that as the semiconductor forbidden gap increases, the maximum glower efliciency is shifted towards the side with higher temperatures and becomes more sharp; the magnitude of the maximum efficiency increases. But this increase in the working temperature requires greater thermal stability of the semiconductor of which the glower is made. That is the reason why semiconductors with excessively large forbidden gaps are ineffective; the optimal width of the forbidden gap of the glower semiconductor proves to be equal approximately to 3 ev. at room temperature. The effiiciency of the glower made of such a semiconductor may be very high at its maximum temperature, i.e. at 2,500" K.

Hexagonal Carborundum has a width of forbidden gap about 3 ev. Therefore it may be used as a material for the glower. Other materials possessing adequate width of the forbidden gap are aluminum nitride, thorium dioxide, and titanium dioxide.

The heating of a tungsten filament by a current passing therethrough presents no difficulties. This is not the case with a strip of semiconductor material. The conductivity of the semiconductor increases in the process of heating. This gives rise to an increase in the current passing therethrough, which, in turn, leads to an increase in the temperature, which again gives rise to the current increase and so on. That is Why, generally speaking, a semiconductor heated by a current may come to an unsteady state.

However, this diflficulty with semiconductor lamps may be prevented. The semiconductor will be in the steady state, if at the working temperature the removal of heat increases more rapidly in the process of heating than its liberation. Then the increase in the Joule heat with the temperature rise cannot support this raised temperature due to the rapid rate of heat removal. This may be attained by alloying the semiconductor with admixtures whose concentration will be sufiicient to impart the admixing character to the conductivity at the working temperature (estimated as it has been said above by the glower optical properties). The concentration amount should be of the order of 10 to 10 cm. The depth of levels created by the admixture, as it has been shown above, should not exceed 0.5 ev. As such an admixture for Carborundum is boron. If it is desirable that the lamp be ignited independently without additional heating it is necessary that the conductivity of the glower at room temperature be sufliciently high. If the admixture creates levels as deep as 0.3 ev. the lamp will be ignited if the ambient temperature is approximately 0 C. Such a level in the Carborundum may be achieved by the aid of aluminum. If the depth of admixture levels is 0.15 ev., the lamp may be ignited even at 70 C. The levels of such a depth in the Carborundum are created by the aid of nitrogen and phosphorus.

In an electric lamp with a glower of semiconductor material, the energy losses due to the heat conduction amount to about one half of all the energy applied thereto. Hence, a further increase in the luminous efficiency of such an incandescent lamp is possible due to reducing the energy losses stemming from the heat conduction. If the lamp glower is placed in a flask, with a double wall with vacuum therebetween, in such a manner as in the Dewar flask, it is possible to decrease the heat losses by 20 to 40. This increases the luminous efiiciency by 10 to 20%.

Such a lamp according to the invention is shown in FIG. 4 wherein a glower 1 of semiconductor material is placed within a vessel having spaced walls 2 with a vacuum established therebetween.

The use of flasks with double walls in lamps with a tungsten filament proves to be inexpedient as the increase in the luminous efficacy brought about by decreasing the heat conduction will be insignificant.

What is claimed is:

1. In an electric incandescent lamp a glower made of semiconductor material alloyed with admixtures selected from the group consisting of donor and acceptor admixtures in a concentration of 10 to 10 cm.- and providing in said semiconductor material energy levels not deeper than 0.5 electron volt.

2. In the electric incandescent lamp 3. glower, according to claim 1, wherein the donor and acceptor admixtures provide energy levels of 0.3 electron volt.

3. In the electric incandescent lamp a glower, according to claim 1, made of Carborundum alloyed with nitrogen.

4. In the electric incandescent lamp a glower, according to claim 1, made of Carborundum alloyed with phosphorus.

5. In the electric incandescent lamp a glower, according to claim 1, made of Carborundum alloyed with nitrogen and phosphorus.

6. In the electric incandescent lamp a glower, according to claim 1, made of Carborundum alloyed with aluminum.

7. In the electric incandescent lamp a glower, according to claim 2, made of Carborundum alloyed with boron.

8. An electric incandescent lamp comprising a vessel with spaced walls defining a vacuum therebetween, a glower in said vessel made of a semiconductor material alloyed with admixtures selected from the group consisting of donor and acceptor admixtures in concentrations of 10 to 10 cm. and providing in said semiconductor material energy levels not deeper than 0.5 electron volt.

9. An electric incandescent lamp, according to claim 5 6 8, in which the donor and acceptor admixtures provide 2,784,284 3/1957 Zunick 313-334 X in said semiconductor energy levels of 0.3 electron volt. 2,870,520 1/1959 Desvignes 313-334 X 2,923,849 2/1960 Rees 313-311 References JOHN W. HUCKERT, Primary Examiner UNITED STATES PATENTS 5 ANDREW J. JAMES, Assistant Examiner 1,148,184 7/1915 Mott 313-311 1,401,510 12/1921 Baumhauer 313-218 X US. Cl. X.R.

2,005,897 6/1935 Knowles 313-334 X 313-311, 315 

